This invention relates generally to neutral beam injection systems for use in controlled nuclear fusion devices such as Tokomaks, magnetic mirror systems, bumpy tori and the like, and, more particularly, this invention relates to improvements in neutral beam injectors with direct ion energy recovery.
In the field of controlled nuclear fusion, a high temperature plasma is formed of fusionable light isotope ions contained within a magnetic field confinement or containment zone, in an evacuated region. Such plasma may generally comprise one or more isotopes, such as hydrogen, deuterium, tritium, helium 3, etc., which undergo fusion reactions under appropriate conditions of confinement time, density, and temperature. These conditions may be brought about or supplemented by the injection of properly accelerated neutral particle beams of one or more of the appropriate species into the magnetically confined plasma. The injected particles must be neutral in order to penetrate the very strong magnetic field. These energetic neutral particles are subsequently ionized either by collision with the plasma ions or by action of the magnetic confinement field (Lorentz force), and are accordingly trapped to increase the plasma temperature in the magnetic containment zone.
Since the neutral particles cannot be directly accelerated to high velocity, i.e., high kinetic energies, they are produced in an indirect manner from an ion source. It has been the practice to produce a neutral beam by accelerating either positively or negatively charged ions of one or more of the above species emerging from an ion source, and by transporting them through a gas-cell neutralizer wherein they interact with background neutral atoms of the same species at a specified pressure through charge-exchange collisions. A certain portion of the energetic ions is converted into neutrals (or neutralized) as they emerge from the neutralizer. The ion-to-neutral conversion efficiency depends on the species and the beam energy. The positive ion-to-neutral efficiency decreases monotonically as the energy per nucleon increases, e.g., from 60% for 40 kilo electron volts (keV) to 15% for 100 keV per nucleon.
As future ion sources are developed toward energies of about 100 keV per nucleon, the efficiency of producing energetic neutral hydrogen isotopes from positive ions is intolerably low.
One way to improve the neutral beam injector efficiency is to recover the energy contained in the unneutralized residual ion fraction of the beam, which is otherwise wasted. In order to recover the kinetic energy of the unneutralized residual ion component of the beam emerging from the neutralizer cell in the form of usable electric energy, the beam ions must be diverted from the neutral beam passage, decelerated, and collected. The electrons present in the neutralizer cell must be blocked from entering the ion collector area since they would be accelerated into it, thereby producing an energy loss which may be equal to or greater than the recovered ion energy.
In the process of developing direct energy recovery in neutral beam injectors, various means have been devised or suggested, which may be generally divided into two groups, depending upon the ion deflection method used. They are either electrostatic or magnetic ion deflection methods.
An electrostatic deflection system is described in a paper by W. L. Barr et al, "Proceedings of 7th Symposium on Engineering Problems of Fusion Research", Vol. 1, page 308, 1978. This paper discloses an electrostatic system developed at Lawrence Livermore National Laboratory, Livermore, Calif., in which the neutralizer cell wall is held at ground potential, the ion beam collector is biased highly positive up to the initial beam energy, and the electrons emerging from the cell are repelled by a negative voltage (approximately 20 kV) applied to a set of electrodes which closely encompass the beam. One negative electrode is placed between the neutralizer cell exit and a funnel-shaped ion collector which also encompasses the beam. The other negative electrode is placed at the exit of the collector. The ion collector acts to decelerate and collect the ions diverging radially from the beam. For a successful electron blocking in this system, the negative electrodes must be biased sufficiently negative to drive the beam potential negative even on the axis in the presence of the positive-ion space-charge and the nearby positive-ion collector. There are inherent problems with this system which include a severe vacuum requirement for efficient direct conversion, increased beam-line length needed to collect most of the diverging and decelerating ions which consequently reduces the neutral power transmission efficiency, and uncertain trajectories of fractional energy ions. Low gas pressure is critically required since the slow ions and electrons produced by ionization and charge exchange of the background gas are likely to be drawn to these electrodes causing excessive power loading. The subsequent emission of secondary electrons from the surface of the negative electrodes would give rise to an additional power drain.
Other electrostatic electron-blocking and ion-collection systems utilizing electrostatic grids are discussed by P. Raimbault in EUR-CEA-FE-823, 1976. One specific system outlined in this reference employs a cylindrical grid arrangement which surrounds the beam exiting the neutralizer which is biased negative with respect to the neutralizer to suppress the electrons. This long cylindrical suppression grid is supposed to ease the higher negative voltage required to penetrate into the beam and to block the electrons. However, this scheme also suffers from direct interception of the ion beam on the negative potential, cylindrical grid. Not only is the ion energy lost, but secondary electrons ejected from the grid by the ion impingement constitute an additional power loss. In this system, unlike the former system, the ion source is operated at near ground potential, and the ions are accelerated by operating the neutralizer at a high-negative potential, which makes it possible to recover the energy of the ions by deceleration to ground potential.
Further, it has been suggested in the art to employ magnetic means for deflecting the ions from the neutral beam, and it has been further suggested to employ magnetic suppression, or blocking, of the electrons from emerging from the neutralizer tube. It has been recognized in the art that magnetic suppression would be advantageous in that the magnetic field can penetrate beams that are too thick and too dense for electrostatic supression to work. However, in the prior-art experiments employing magnetic suppression, there was no provision made to terminate the electrons, nor was there a strong enough magnetic field.
A U.S. Pat. No. 4,349,505, of common assignee with the present invention, filed July 1, 1980 by William L. Stirling for "Neutral Beam Line with Ion Energy Recovery Based on Magnetic Blocking of Electrons" discloses a system employing magnetic blocking of the electrons and electron collection at the neutralizer exit. The neutralizer is operated at a high-negative acceleration potential and the emerging ion beam experiences a strong electric field due to the surrounding ground potential structure which is transverse to the magnetic field applied across the beam at the neutralizer exit. Any electrons present in the beam-generated plasma in the neutralizer are quickly moved out of the beam due to ExB field drift and directed into a slightly positive biased, electron collector. However, there exists a finite fraction of molecular ions (e.g., H.sub.2.sup.+ and H.sub.3.sup.+) along with the atomic ions in the extracted beam from the ion source. These molecular ions are mostly dissociated into atomic particles as they pass through the near-equilibrium gas cell, and thus these dissociated ion particles have kinetic energies of fractional values (one-half or one-third) with respect to the original full acceleration energy (E) of the atomic ions accelerated through the neutralizer and cannot reach the ground-potential surfaces on which the full energy (E) ions are collected. These fractional energy ions are deflected along paths of substantially smaller radius and, in a bad geometry, could impinge upon the outer walls of the neutralizer cell producing secondary electrons which cannot be suppressed from being accelerated to the surrounding ground potential surfaces.
Therefore, it will be appreciated that there is a need for improvements in neutral beamline systems with ion energy recovery based on the advantages of magnetic blocking of electrons and magnetic beam ion deflection. One particular need is to improve the handling of fractional energy ions to prevent their interferring with the recovery of the full energy ions when the fractional energy ions cannot be recovered readily. Further, improvement is needed to prevent full energy ions from prematurely deflecting onto the gas cell wall due to the close proximity of the magnet region to the neutralizer end region.